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I'm not 100% sure if you're looking for an ID on the aphid, or the question is why are these red ones only on Heliopsis and not other plants? There are numerous mentions of red aphids on Heliopsis on the internet but the nearest to an ID I've found is that it may be a Uroleucon species (there are thousands of different aphid species worldwide). This genus is also suggested in BugGuide here:
http://bugguide.net/node/view/...
If the question is why only on Heliopsis, that is likely because plant pests often specialize and have a limited host range, although there are some that seem to be less fussy. So this red aphid species is likely specific to Heliopsis and maybe a few other plants (that you don't have) and does not feed on just any plant that it comes across.
If it's an ID you're looking for you could try posting on the Insect and Bug ID Forum.
